This invention relates to a method of operating an electric power plant, particularly useful for operation of an electric power plant comprising a plurality of combined-cycle power generating units.
Recently, combined-cycle power generating units are widely used for improving the power generating efficiency of a power plant. A combined-cycle power generating unit comprises a gas turbine and a steam turbine which are coupled in tandem such that the heat of the exhaust gas delivered from the gas turbine is used for generating steam for driving the steam turbine. Although the power generating efficiency of the unit is much higher than that of an ordinary thermal power plant because the heat of the exhaust gas of the gas turbine is reused for generating steam, the unit has a shortcoming in that the unit capacity thereof is, at present, not more than a fraction of the unit capacity of a thermal power generating unit.
It is generally desirable that capacity (unit capacity) of a single power plant connected to a power transmission system of a large capacity is large, because building a number of smaller-capacity plants with the same total capacity is more costly and less efficient. A further ground for preference of larger unit capacity plant is the increasing power demand on the power transmission system.
In order to satisfy the two requirements of improving the power generating efficiency and increasing the unit capacity of the power plant, it is advantageous that the power plant is composed of a plurality of combined-cycle power generating units, and that these units are operated in a manner in which the units are treated as a single large-capacity unit, i.e., the plant operator can regard the group of the plural units as a single unit.
Heretofore, methods or devices for controlling a single combined-cycle power generating unit have been developed. However, no methods or devices have been developed for controlling a plurality of combined-cycle power generating units treating them as a single unit. Consequently, the present status of art is that the operation, in each power plant, of a number of combined cycle units has to be made according to human judgement of the operator. This, however, is extremely difficult.
Generally, the power demand on the transmission line is allotted to each power plant by a central dispatching office and the allotment to each power plant varies with time. The allotment to each plant is hereinafter referred to as a "target load". An operator of the power plant comprising a plurality of power generating units carries out start-up and shut-down of the units such that the total electric power generated at every instant by the plant as a whole is equal to the target load. It should be noted, in this connection, that one has to wait certain time after a power generating unit is started and before the control of the power becomes possible. Thus, the operator must start the power generating unit at an instant in advance of the instant at which it is desired to commence control of its output by a time interval corresponding to a start-up time period. However, the estimation of the start-up time period is extremely difficult, and this difficulty is further complicated by the fact that the start-up time period is varied depending on whether the gas turbine and the steam turbine of the particular power generating unit are preheated or not, that is whether the power generating unit is in a "hot mode" or a "cold mode". For example, a unit in the hot mode can start generating an electric power earlier than a unit in the cold mode if the two units are started simultaneously.
For this reason, the burden on an operator of the power plant starting a plurality of combined cycle power generating units for meeting the target load is far severer than that on an operator of a plant comprising a single power generating unit.